1. Field of the Invention
The invention relates to a method and device for adding or extracting a secondary information signal, such as information of a hidden channel for copy protection purposes, to/from a RLL (runlength-limited) code sequence which may be recorded on a record carrier. Furthermore, the invention relates to a record carrier for storing an RLL code sequence and to a binary signal.
2. Description of the Related Art
The invention is applicable to record carriers with general RLL codes. A RLL code is characterized by two parameters, (d+1) and (k+1), which stipulate the minimum and maximum runlength, respectively, that may occur in the coded sequences (within the channel bitstream). For example, the DVD-format uses a (d=2, k=10) RLL code, called EFMPlus.
Additionally, merging bits may be used to insert a transition in order to minimize a low frequency content of the RLL code sequence. Thereby, a DC-control function can be achieved. In CD technology, the use of the 8-to-14 modulation (EFM) code leads to runlength-limited waveforms satisfying the (2,10)-constraint. These waveforms can be parsed into phrases where each phrase corresponds to a land or a pit on the surface of the CD. Each symbol of 8 data bits is mapped onto 14 channel bits. To each block of 14 channel bits, 3 extra bits are added for merging the EFM channel words and for low frequency suppression. The information is contained in the positions of the transitions. Due to the (2, 10)-constraint, the EFM code is generated in such a way that the minimum runlength is 3 channel bits and the maximum runlength is 11 channel bits. Since the three extra merging bits do not contain any information, an extra transition may (or may not) be inserted so as to prevent any violation of the minimum or maximum runlengths.
A limited multi-level modulation (LML modulation) as described in European Patent Application No. 99200873.0, corresponding to U.S. Pat. No. 6,529,147 (PHN 017369) (entitled “Decoding a Limited Multi-Level Channel”), has been proposed to incorporate a secondary channel into the main CD channel. The general idea of the LML modulation code is to use a fine-splitting of the levels of longer runlengths of the RLL code sequence. For example, runlengths of 5 up to 11 channel bits may have a small indention in the middle of the runlength. The presence or absence of this indention corresponds to the value of an extra bit stored in these longer runlengths. The presence of this indention does not disturb the classical detection of the EFM modulation code.
A drawback for LML is that the small indentions for pits (and possibly for lands) are physically observable with appropriate analytical instrumentation that can be used for inspection of the disc (like scanning electron microscopy (SEM), atomic-force microscopy (AFM)). This implies that with appropriate inspection, the area in which the secondary information signal has been written, can directly be observed.